Rare earth magnet and method for producing same

ABSTRACT

A rare-earth magnet includes a magnet body made of an R—Fe—B based rare-earth magnet material (where R is at least one rare-earth element) and a metal film that has been deposited on the surface of the magnet body. The magnet further includes a plurality of reaction layers between the magnet body and the metal film. The reaction layers include: a first reaction layer, which contacts with at least some of R 2 Fe 14 B type crystals, included in the magnet body, to have received the rare-earth element that has been included in the R 2 Fe 14 B type crystals; and a second reaction layer, which is located between the first reaction layer and the metal film and which has a lower rare-earth element concentration than that of the first reaction layer.

This application is a Divisional of S1. Ser. No. 12/145,595 filed Jun.25, 2008, now U.S. Pat. No. 7,655,325, which is a continuation ofPCT/JP2006/325856 filed Dec. 26, 2006, and claims priority over JapaneseApplication 2005-378905 filed Dec. 28, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rare-earth magnet and a method forproducing the magnet.

2. Description of the Related Art

An R—Fe—B based rare-earth sintered magnet is known as a magnet with thehighest performance among various types of permanent magnets, and hasbeen used extensively in a voice coil motor (VCM) for a hard disk driveand in a magnetic circuit for a magnetic resistance imaging (MRI), forexample.

In the prior art, an R—Fe—B based sintered magnet embedded in a magneticcircuit has a weight of about 100 g to about 1 kg each. Depending on theapplications, some big magnets may have a weight exceeding 1 kg.Recently, however, small-sized sintered magnets have been used more andmore often in optical pickups and motors of very small sizes, forexample. Some of those small-sized sintered magnets may have a weight ofless than 1 g.

A magnetic circuit that adopts such small-sized and lightweight magnetsneeds to keep the performance of a final product high enough whilemeeting the demands for reducing the size and weight of the finalproduct. That is why magnets for use in such a magnetic circuit shouldexhibit strong magnetic properties even though their size is small.Therefore, there have been growing demands for high-performance R—Fe—Bbased sintered magnets in the field of small-sized magnets, too.

It is known that the coercivity of an Nd—Fe—B based magnet is producedby its internal nanostructure in which an Nd₂Fe₁₄B main phase issurrounded with thin Nd-rich phases, thus realizing a high maximumenergy product.

However, when an Nd—Fe—B based sintered magnet is actually used inmotors, for example, the magnet is usually subjected to a grindingprocess to finish it in a desired final size and to obtain apredetermined degree of concentricity. During that process, the Nd-richphase on the surface layer of the magnet is often damaged due to verysmall grinding cracks or oxidation. As a result, the magnetic propertieson the surface portion of the magnet may decrease to only a fraction ofthe properties inside the magnet.

This phenomenon is observed particularly noticeably in small-sizedmagnets with a large surface area to volume ratio. For example, if ablock magnet of 10 mm square, having a (BH)_(max) of 360 kJ/m³, is cutinto rectangular parallelepiped shapes with dimensions of 1 mm×1 mm×2 mmand then subjected to a grinding process, then their (BH)_(max) willdecrease to about 240 kJ/m³. As a result, the essential properties ofthe Nd—Fe—B based rare-earth magnet are not realized anymore.

Also, as a result of the machining process, a machine-degraded layerhaving no coercivity anymore is always formed on the surface of thesintered magnet. Since the coercivity of the magnet has been lost fromsuch a machine-degraded layer, that layer will not function as a magneteven when magnetized. If the sintered magnet has a sufficiently largevolume, such a machine-degraded layer, if ever, accounts for just asmall volume percentage. Thus, the overall performance of the magnet ishardly affected by the machine-degraded layer. However, if the sinteredmagnet has a decreased volume, then the machine-degraded layer accountsfor an increased volume percentage. In that case, its influence isnon-negligible.

Suppose the volume of the machined sintered magnet is V, the totalsurface area of the sintered magnet is S, and the thickness of themachine-degraded layer is d. In that case, the volume of themachine-degraded layer is approximated at Sd. Therefore, the volumepercentage of the machine-degraded layer to the overall sintered magnetwith the volume V is Sd/V. The volume of the intact portion of thesintered magnet, still maintaining coercivity, is given by V-Sd. Thus,the remanence of the overall machined magnet is obtained by multiplyingits original value (i.e., before the machining process) by(V−Sd)/V=1−Sd/V. That is to say, the Sd/V value of a magnet becomes anindex indicating how much the machine-degraded layer of the magnetaffects its magnetic properties.

Sd/V is the product of S/V and d. The former is a factor determined onlyby the shape of the magnet, whereas the latter is a factor determined bythe process history of the machine-degraded layer. The smaller thevolume of a magnet, the larger S/V becomes and the smaller d should be.

Hereinafter, conventional techniques of doing some type of processing onthe surface of a sintered magnet will be described.

Patent Document No. 1 discloses a permanent magnet material, in which arare-earth metal such as Nd, Pr, Dy, Ho, Tb, La, Ce, Sm, Gd, Er, Eu, Tm,Yb, Lu or Y has been deposited on the surface to be machined and turnedinto a reformed layer through a diffusion process.

Patent Documents Nos. 2 and 3 teach forming a film of titanium metal ora titanium compound such as titanium, a titanium nitride, a titaniumcarbide or a titanium oxide on the surface of a rare-earth-iron basedmagnet.

Patent Document No. 4 proposes providing a coating of a compoundincluding Ti and at least one element selected from the group consistingof Nd, Fe, B and O.

Patent Document No. 5 teaches forming a thin-film layer, consistingessentially of Sm and Co, on the ground surface of an Nd—Fe—B basedsintered magnet that has been subjected to a grinding process.

Patent Document No. 6 teaches coating the surface of a machined magnetwith a refractory metal (which may be Ta with a particle size of 100 μmor less according to a working example) and also proposes embedding themagnet in particles of the refractory metal and dissolving them at atemperature of 700° C. to 900° C.

Patent Document No. 7 discloses a method of improving the loopsquareness by depositing Pd or a Pd metal layer on the surface of amachined magnet by an evaporation process, for example, and then meltingthe machine-degraded layer with a laser beam. Pd is used to get theplating process done more easily.

Patent Document No. 8 discloses a rare-earth magnet that has beenmachined so as to have an S/V value of 2 mm⁻¹ or more and a volume of100 mm³ or less. According to Patent Document No. 8, to reform adegraded and damaged portion formed by a machining process, a rare-earthmetal is diffused from the surface of the magnet so as to penetratedeeper than the radius of crystal grains that are exposed on the surfaceof the magnet.

-   -   Patent Document No. 1: Japanese Patent Application Laid-Open        Publication No. 62-74048    -   Patent Document No. 2: Japanese Patent Application Laid-Open        Publication No. 63-9908    -   Patent Document No. 3: Japanese Patent Application Laid-Open        Publication No. 63-9919    -   Patent Document No. 4: Japanese Patent Application Laid-Open        Publication No. 63-168009    -   Patent Document No. 5: Japanese Patent Application Laid-Open        Publication No. 2001-93715    -   Patent Document No. 6: Japanese Patent Application Laid-Open        Publication No. 2001-196209    -   Patent Document No. 7: Japanese Patent Application Laid-Open        Publication No. 2002-212602    -   Patent Document No. 8: Japanese Patent Application Laid-Open        Publication No. 2004-304038

Recently, there are growing demands for ultra small magnets. The demandsare escalating not just in the fields of optical pickups and ultra smallmotors but also in the fields of cardiosurgery and neurosurgery as well.In the fields of these cutting-edge medical treatments, a technique forcontrolling the direction in which a vascular catheter advances at abranching point of a blood vessel by attaching a small high-performancemagnet to the end of the catheter and applying a magnetic field fromoutside of the patient's body has been researched. On the other hand, ina magnetic induction surgical system, it has been proposed that an ultrasmall magnet be embedded at a particular location of the body and usedas a location marker. The ultra small magnets for use in suchapplications should have a cylindrical shape with a diameter of 0.3 mmand a length of 2 mm, for example. In that case, the S/V value exceeds10 mm⁻¹. Such a magnet needs to have magnetic properties that are highenough to make the magnet work fine irrespective of its small size.

If the size of a magnet is reduced, however, the performance of themagnet, which would be much higher if the magnet had a big size, may notbe exhibited fully.

Patent Document No. 1 proposes coating the machine-degraded layer on theground surface of a sintered magnet with a thin rare-earth metal layerto make a reformed layer through a diffusion reaction. Morespecifically, Patent Document No. 1 discloses an experimental example inwhich a sputtered film is formed on a thin test piece with a length of20 mm, a width of 5 mm and a thickness of 0.15 mm but achieves a(BH)_(max) of only 200 kJ/m³ at most. Also, the surface is oxidizedduring the diffusion process by annealing, thus causing inconvenience inthe subsequent surface treatment.

Patent Documents Nos. 2, 3 and 4 disclose techniques for increasing thecorrosion resistance of a rare-earth-iron based magnet to be corrodedeasily but are silent about how to repair the degradation caused bymachining.

According to the technique disclosed in Patent Document No. 5, Sm,diffusing into the magnet as a result of the heat treatment process,deteriorates the magnetic anisotropy of the Nd₂Fe₁₄B phase crystals.

According to the technique disclosed in Patent Document No. 6, as longas heat treatment is carried out, it is difficult to minimize theoxidation at the surface of a rare-earth magnet. Consequently, it isalso difficult to recover the properties just as intended.

In the method disclosed in Patent Document No. 7, it is not costeffective to deposit Pd metal or melt the machine-degraded layer with alaser beam.

Patent Document No. 8 reports that by depositing a heavy rare-earthelement Dy or Tb by a sputtering process and diffusing the element intothe mother phase, not only can the magnetic properties be recovered butalso can the coercivity be increased significantly. However, the heattreatment process adopted in this method is not cost effective becauseit is necessary to control the concentration of oxygen in the atmosphereand the dew point thereof with high precision. Besides, this methodlacks mass-productivity because a lot of magnets cannot be processed ina single batch.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, the present inventionprovides a rare-earth magnet with minimal deterioration in properties atthe surface and a method for producing such a magnet.

A rare-earth magnet according to a preferred embodiment of the presentinvention includes a magnet body made of an R—Fe—B based rare-earthmagnet material (where R is at least one rare-earth element) and a metalfilm that has been deposited on the surface of the magnet body. Themagnet further includes a plurality of reaction layers between themagnet body and the metal film. The reaction layers include: a firstreaction layer, which contacts with at least some of R₂Fe₁₄B typecrystals, included in the magnet body, to have received the rare-earthelement that has been included in the R₂Fe₁₄B type crystals; and asecond reaction layer, which is located between the first reaction layerand the metal film and which has a lower rare-earth elementconcentration than that of the first reaction layer.

In one preferred embodiment, the second reaction layer has boron thathas been included in the R₂Fe₁₄B type crystals and has a higher boronconcentration than that of the first reaction layer.

In another preferred embodiment, the rare-earth element concentration ofthe first reaction layer accounts for at least 30 mass % of the entirecomposition of the first reaction layer.

In still another preferred embodiment, the first reaction layer has athickness of at least 10 nm.

In yet another preferred embodiment, the magnet body has a surface areato volume ratio of at least 2 mm⁻¹ and a volume of at most 100 mm³.

In yet another preferred embodiment, the surface of the magnet bodyincludes a surface area that has been formed by a machining process andthat is covered with the metal film.

In yet another preferred embodiment, the metal film is made of at leastone metal that is selected from the group consisting of Ti, V, Zr, Nb,Mo, Hf, Ta and W or an alloy thereof.

In yet another preferred embodiment, the surface area of the magnet bodyhas a surface roughness Ra of 0.5 μm or less.

In yet another preferred embodiment, the rare-earth magnet furtherincludes an anti-corrosive coating that has been formed so as to coverthe metal film.

A method for producing a rare-earth magnet according to anotherpreferred embodiment of the present invention includes the steps of:providing a magnet body made of an R—Fe—B based rare-earth magnetmaterial (where R is at least one rare-earth element); and depositing ametal film on the surface of the magnet body. The method furtherincludes a heat treatment process step that is performed to form,between the magnet body and the metal film, a plurality of reactionlayers including: a first reaction layer, which contacts with at leastsome of R₂Fe₁₄B type crystals, included in the magnet body, to havereceived the rare-earth element that has been included in the R₂Fe₁₄Btype crystals; and a second reaction layer, which is located between thefirst reaction layer and the metal film and which has a lower rare-earthelement concentration than that of the first reaction layer.

In one preferred embodiment, the second reaction layer has receivedboron that has been included in the R₂Fe₁₄B type crystals and has ahigher boron concentration than that of the first reaction layer.

In another preferred embodiment, the rare-earth element concentration ofthe first reaction layer accounts for at least 30 mass % of the entirecomposition of the first reaction layer.

In still another preferred embodiment, the first reaction layer has athickness of at least 10 nm.

In yet another preferred embodiment, the magnet body has a surface areato volume ratio of at least 2 mm⁻¹ and a volume of at most 100 mm³.

In yet another preferred embodiment, the surface of the magnet bodyincludes a surface area that has been formed by a machining process andthat is covered with the metal film.

In yet another preferred embodiment, the metal film is made of at leastone metal that is selected from the group consisting of Ti, V, Zr, Nb,Mo, Hf, Ta and W or an alloy thereof.

In yet another preferred embodiment, the surface area of the magnet bodyhas a surface roughness Ra of 0.5 μm or less.

Various preferred embodiments of the present invention provide arare-earth permanent magnet, of which the degradation in magneticproperties at the surface can be minimized and which can exhibitexcellent corrosion resistance even under a harsh environment.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view schematically illustrating thenanostructure of a rare-earth sintered magnet, FIG. 1B is across-sectional view of the sintered magnet of which the surface hasbeen subjected to a machining process, and FIG. 1C is a cross-sectionalview of a sintered magnet in which a metal film and reaction layers havebeen formed on its surface.

FIG. 2 is a cross-sectional view that was drawn based on a TEMphotograph of a specific example of preferred embodiments of the presentinvention.

FIG. 3 is a graph showing how much the coercivity increased or decreasedwith the heat treatment temperature.

FIG. 4 is a graph showing how much the coercivity increased or decreasedwith the surface roughness Ra of a magnet body on which no Ti film hadbeen deposited yet.

FIG. 5 is a graph showing the respective demagnetization curves of amagnet test piece on which a Ti film was deposited and which wasthermally treated, a magnet test piece on which a Ti film was depositedbut which was not thermally treated, and a magnet test piece on which noTi film was deposited and which was just machined.

FIG. 6 is a graph showing the respective demagnetization curves of amagnet test piece on which a V film was deposited and which wasthermally treated, a magnet test piece on which a V film was depositedbut which was not thermally treated, and a magnet test piece on which noV film was deposited and which was just machined.

FIG. 7 is a graph showing the respective demagnetization curves of amagnet test piece on which a Zr film was deposited and which wasthermally treated, a magnet test piece on which a Zr film was depositedbut which was not thermally treated, and a magnet test piece on which noZr film was deposited and which was just machined.

FIG. 8 is a graph showing the respective demagnetization curves of amagnet test piece on which a Nb film was deposited and which wasthermally treated, a magnet test piece on which a Nb film was depositedbut which was not thermally treated, and a magnet test piece on which noNb film was deposited and which was just machined.

FIG. 9 is a graph showing the respective demagnetization curves of amagnet test piece on which a Mo film was deposited and which wasthermally treated, a magnet test piece on which a Mo film was depositedbut which was not thermally treated, and a magnet test piece on which noMo film was deposited and which was just machined.

FIG. 10 is a graph showing the respective demagnetization curves of amagnet test piece on which a Hf film was deposited and which wasthermally treated, a magnet test piece on which a Hf film was depositedbut which was not thermally treated, and a magnet test piece on which noHf film was deposited and which was just machined.

FIG. 11 is a graph showing the respective demagnetization curves of amagnet test piece on which a Ta film was deposited and which wasthermally treated, a magnet test piece on which a Ta film was depositedbut which was not thermally treated, and a magnet test piece on which noTa film was deposited and which was just machined.

FIG. 12 is a graph showing the respective demagnetization curves of amagnet test piece on which a W film was deposited and which wasthermally treated, a magnet test piece on which a W film was depositedbut which was not thermally treated, and a magnet test piece on which noW film was deposited and which was just machined.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors discovered that if the surface of a very smallmagnet, which had been produced by subjecting a block of a rare-earthsintered magnet to cutting, boring, grinding, polishing or any othermachining process, was reformed, then the degradation in magneticproperties at the surface could be repaired and the corrosion resistancecould also be improved, thus perfecting the present invention.

According to a preferred embodiment of the present invention, a film ofa particular metal is deposited on the surface of a magnet body and thensubjected to a heat treatment process, thereby forming a reaction layerwith a two-layer structure. Thanks to the function of this reactionlayer, the magnetic properties at the surface of the magnet body can beimproved.

A magnet body for use in preferred embodiments of the present inventionmay have any composition as long as the magnet can be turned into adesired shape by processing a bulk magnet material. Its manufacturingprocess is not limited to any particular one, either. However, thepresent invention is applicable most effectively to magnets that have acoercivity producing mechanism of a nucleation type. Typical examples ofthe magnets of this type include R—Fe—B based sintered magnets and 1-5Sm—Co sintered magnets. Among other things, the R—Fe—B based sinteredmagnets have excellent machinability and can be processed into a shapeof a very small size relatively easily. In R—Fe—B based sintered magnet,R is at least one rare-earth element and preferably includes at leastone of Nd and Pr as an essential element and may include Dy and/or Tb ifnecessary. As a specific composition, a magnet composition that iscurrently used extensively as an Nd—Fe—B based magnet may be adopted. Inthe magnets of such a material, the balance between the flux density andthe coercivity can be adjusted by changing the percentages of Ndreplaced with Dy or Tb in the rare-earth element R.

An R—Fe—B based sintered magnet, having a known composition such as thatdisclosed in Japanese Patent Gazette for Opposition No. 59-64733, forexample, is preferably used as the magnet body according to a preferredembodiment of the present invention. The R—Fe—B based sintered magnethas a composition consisting essentially of a rare-earth element R,boron B and Fe. More specifically, the magnet includes 8 at % to 30 at %of R, 2 at % to 28 at % of B, and the balance consisting essentially ofFe. A portion (at most 50%) of Fe may be replaced with Co. Also, aportion of B may be replaced with carbon C. In the rare-earth elementsR, the sum of Dy and Tb preferably accounts for at least 0.3 at % of allrare-earth elements R and the balance is preferably Nd and/or Pr. A morepreferable composition includes 13 at % to 15 at % of R and 5.5 at % to7 at % of B.

The sintered magnet body described above may be produced by performingvarious powder metallurgical manufacturing process steps includingmelting a material alloy, pulverizing the alloy, compacting the powderunder a magnetic field, sintering the green compact, and subjecting thesintered compact to an aging treatment.

According to a preferred embodiment of the present invention, thesurface of the magnet body is preferably planarized as much as possiblebefore the metal film is deposited thereon. Typical examples of themachining processes that can be adopted in preferred embodiments of thepresent invention include wire sawing, blade sawing, surface grindingand lapping. However, the present invention is in no way limited tothese specific examples but any other machining process may be adoptedas well. It should be noted that a rare-earth sintered magnet materialis so brittle as to cause grain detachment easily during the machiningprocess. If too many grains were detached from the magnet material, nouniform reaction layers could be formed and the magnetic propertieswould not be recovered even when the metal film is deposited on themachined surface. That is why before the metal film is deposited, themachined surface has preferably been finished so as to avoid graindetachment. Also, the machined surface preferably has as little oxide aspossible.

Hereinafter, it will be described with reference to FIGS. 1A through 1Cwhy the magnet performance is improved by the structure of the presentinvention.

FIG. 1A is a partial cross-sectional view schematically illustrating theinternal nanostructure of an R—Fe—B based rare-earth sintered magnet.FIG. 1B is a partial cross-sectional view of the sintered magnet ofwhich the surface has been subjected to a machining process. And FIG. 1Cis a partial cross-sectional view of a magnet according to a preferredembodiment of the present invention in which a metal film and reactionlayers have been formed on its surface.

As shown in FIG. 1A, inside the R—Fe—B based rare-earth sintered magnet,R₂Fe₁₄B type crystal grains 10 as a main phase are surrounded with agrain boundary phase (auxiliary phase) 12, and those crystal grains 10are separated from each other by the grain boundary phase 12. It isknown that in an R₂Fe₁₄B based rare-earth sintered magnet, the grainboundary phase 12 plays an important role in producing the coercivity asdescribed above. The grain boundary phase 12 includes an R-rich phase(not shown) that has a higher rare-earth element concentration than thatof the main phase 10.

If the rare-earth sintered magnet is subjected to a machining process toexpose the surface 100 of the magnet body, then some main phases (i.e.,R₂Fe₁₄B type crystals) that are not covered with the grain boundaryphase 12 are present on the surface 100 of the magnet body (i.e., on themachined surface). The grain boundary phase 12 plays a very importantrole in producing the coercivity as described above. However, as thesurface 100 of the magnet body is not covered with the grain boundaryphase 12 as shown in FIG. 1B, the magnetic properties will deteriorateat and near the surface 100 of the magnet body.

On the other hand, according to a preferred embodiment of the presentinvention, the magnetic properties could be improved by not justcovering the surface of the magnet body 100 with a metal film 20 butalso forming special reaction layers 21 and 22 as shown in FIG. 1C.These reaction layers 21 and 22 are believed to perform a similarfunction to that of the internal grain boundary phase 12.

The properties and thicknesses of the reaction layers 21 and 22 changeaccording to the type of the metal film 20 deposited and the heattreatment temperature. The present inventors discovered and confirmedvia experiments that when a metal film 20 made of at least one metalselected from the group consisting of Ti, V, Zr, Nb, Mo, Hf, Ta and W oran alloy thereof was deposited and when the heat treatment was conductedat a temperature of 400° C. to 750° C., most preferred reaction layers21 and 22 could be obtained.

The metallic material such as Ti cited above tends to be easily bondedto iron or boron in a rare-earth sintered magnet by nature. Thus, thepresent inventors discovered that if a film of the metal was depositedon the surface 100 of the magnet body and then thermally treated underproper conditions, the metal film 20 reacted with the surface 100 of themagnet body to form at least two reaction layers 21 and 22 havingmutually different compositions in the thickness direction.

The first reaction layer 21 contacts with at least a portion of the mainphase (i.e., R₂Fe₁₄B type crystals) 10 included in the magnet body andhas received the rare-earth element that has been included in theR₂Fe₁₄B type crystals. The rare-earth element concentration of the firstreaction layer 21 preferably accounts for at least 30 mass %, and morepreferably 35 mass % or more, of the entire composition of the firstreaction layer 21. When the rare-earth element has such a highconcentration in the first reaction layer 21, the first reaction layer21 will function in the same way as the R-rich phase that is located onthe grain boundary between the main phases (R₂Fe₁₄B type crystals) 10 ofthe rare-earth sintered magnet. However, the present inventors alsodiscovered that when the rare-earth element concentration was too low,the first reaction layer 21 functioned as a ferromagnetic layer todeteriorate the magnetic properties rather than improving them.

As described above, the grain boundary phase 12 plays a key role inproducing coercivity in a rare-earth sintered magnet. However, if thesurface of the main phase (R₂Fe₁₄B type crystals) 10 that wouldotherwise be exposed is covered with the first reaction layer 21 on thesurface 100 of the magnet body, then the first reaction layer 21functions in the same way as the grain boundary phase 12, thusminimizing the decrease in coercivity at and near the surface 100 of themagnet body.

To achieve such an effect of maintaining the coercivity, the firstreaction layer 21 is preferably as thick as the grain boundary phase 12of the rare-earth sintered magnet and preferably has a thickness of 10nm to 200 nm, for example. It should be noted that it is important todeposit the first reaction layer 21 as a continuous film that neverdiscontinues at the interface.

On the other hand, the second reaction layer 22 is located between thefirst reaction layer 21 and the metal film 20 and has a lower rare-earthelement concentration than that of the first reaction layer 21. Thesecond reaction layer 22 includes boron that has been included in themain phase (i.e., the R₂Fe₁₄B type crystals) 10 and has a higher boronconcentration than that of the first reaction layer 21. The secondreaction layer 22 is formed as a result of an increase in boronconcentration while the metal film 20 described above is reacting withthe surface 100 of the magnet body because constituent atoms of themetal film 20 are easily bonded to boron included in the magnet body bynature. By forming the second reaction layer 22 by taking advantage ofsuch a property of the metal, the first reaction layer 21 with a higherrare-earth element concentration (than the main phase 10) can beprovided between the second reaction layer 22 and the main phase 10.That is to say, if a heat treatment process is carried out at anappropriate temperature after a film of a metallic material that easilyreacts with boron has been deposited, a layer with an increased boronconcentration (i.e., the second reaction layer 22) and a layer with anincreased rare-earth element concentration (i.e., the first reactionlayer 21) can be formed substantially simultaneously.

If the metal film 20 deposited were too thick, then the flux densitywould decrease. For that reason, the thickness of the metal film 20 ispreferably defined within such a range in which the decrease in fluxdensity can be no greater than approximately 1%. The upper limit of thethickness of the metal film 20 may be 10 μm, for example. Nevertheless,if the metal film 20 were too thin, the reaction layers described abovecould not be formed. That is why the metal film 20 preferably has athickness of at least 0.5 μm. More preferably, the metal film 20 has athickness of 1 μm to 3 μm.

Before the metal film 20 is deposited, the machined surface 100 of themagnet body may be subjected to a known purifying process such ascleaning, degreasing or inverse sputtering. It should be noted that themetal film 20 to be deposited does not have to have a single layerstructure but may also have a multilayer structure consisting of metallayers of multiple different types. In the latter case, the layers thatplay the most important role are the lowermost layer that contacts withthe machined surface of the magnet body and the uppermost layer that isexposed to the air. The material of the lowermost layer is selected inorder to recover the magnetic properties, while that of the uppermostlayer may be selected from the standpoint of corrosion resistance. Amongvarious metallic materials, Ti is highly recommended because Ti not justcontributes to recovering the magnetic properties but also improves thecorrosion resistance and does no harm to human bodies. When a very smallmagnet is used for medical purposes, Ti can be a very good coatingmaterial.

Hereinafter, preferred embodiments of the present invention will bedescribed.

First, an Nd—Fe—B based rare-earth sintered magnet is produced by aknown manufacturing process. This rare-earth sintered magnet can be madeby sintering a green compact of a rare-earth magnet powder. By machiningthe rare-earth sintered magnet thus obtained, a magnet body can be madein desired shape and size.

The magnets to which preferred embodiments of the present invention areapplied are often used in small-sized actuators and motors, andtherefore, frequently are required to have a high flux density torealize high torque. For that reason, in those applications, therare-earth element R may include either no Dy at all or a very smallamount of Dy if ever. On the other hand, if Dy and/or Tb are/iscontained, the coercivity can be increased significantly by depositingthe metal film. Thus, to increase the coercivity, the magnets preferablyinclude less than 2 mass % of Dy and/or Tb in the overall composition.

Subsequently, a small magnet body with an S/V ratio (which is the ratioof the surface area S (mm²) to the volume V (mm³)) of 2 mm⁻¹ or more iscut out of the magnet body. Next, the surface of the magnet body isprocessed by a barrel polishing process or any other suitable process,thereby smoothing the surface to a surface roughness Ra of 0.5 μm.

Thereafter, a metal film is deposited on the surface of the magnet body.A wet deposition process such as plating produces hydrogen that woulddeteriorate the performance of the magnet, and therefore, a drydeposition process is preferred. Dry deposition processes areclassifiable into physical vapor deposition (PVD) processes and chemicalvapor deposition (CVD) processes. The PVD processes include vaporizingtypes and sputtering types. Examples of the former types includeevaporation, ion plating, arc ion plating, hollow cathode ion plating,and ion beam evaporation. As a sputtering type, a magnetron sputteringis generally adopted. Examples of the CVD processes include thermal CVD,plasma CVD, optical CVD and MOCVD. Any of these methods may be adopted.Considering the deposition rate and the maintainability of the system,however, ion plating or magnetron sputtering is preferably adopted.Optionally, to increase the degree of close contact between the metalfilm and the magnet body, the magnet body may be heated to a temperatureof 200° C. to 400° C. when the metal film is deposited.

In a preferred embodiment of the present invention, after a metal filmhas been deposited on the surface of the magnet body, a heat treatmentis carried out at a temperature of 400° C. to 1,000° C., preferably at atemperature of 500° C. to 750° C. To advance the interfacial reaction,the magnet needs to be heated to 400° C. or more. The heat treatmentprocess time changes with the heat treatment temperature. For example,if the heat treatment temperature is 400° C., the heat treatment processtime may be three to five hours. As the heat treatment temperaturerises, the heat treatment process time can be shortened. However, if theheat treatment temperature exceeded 1,000° C., the magnet structurecould change and it would become difficult to control the formation ofthe reaction layers. Optionally, part or all of the reaction layers maybe formed while the metal film is being deposited. In any case, themagnet needs to be heated to a temperature of 400° C. or more that ishigher than the maximum allowable temperature for a normal thin-filmdeposition system, the heater in the thin-film deposition system shouldhave an increased size.

After the metal film has been deposited and after the reaction layershave been formed, an anticorrosive coating may be further formed on themetal film. Although the corrosion resistance has already been increasedby the metal film, the magnet should have particularly high surfacepurity especially when used in a voice coil motor (VCM) for a hard diskdrive. That is why a Ni film is preferably formed on the uppermostsurface by electrical plating process. On the other hand, ifadhesiveness is required, the uppermost surface is preferably coatedwith Al.

As described above, the present invention has an effect of minimizingthe degradation in magnetic properties at the surface and can achieve asignificant effect in a small magnet that has a large surface area S forits volume V. A magnet body on which the effect of the present inventioncan be achieved most significantly is an ultra small magnet with an S/Vratio of 2 mm⁻¹ or more and a volume V of 100 mm³ or less. For example,a cubic magnet with a size of 2 mm each side has a surface area of 24mm² and a volume of 8 mm³. Thus, its surface area to volume ratio is 3mm⁻¹. Compared to a cubic magnet with the same volume, a cylindricalmagnet has a greater surface area to volume ratio. In such an ultrasmall magnet, if the magnetic properties deteriorated at the surface,its influence on the overall magnet performance would be anon-negligible one. Therefore, if a machine-degraded layer were formedon the surface of an ultra small magnet, then the loop squareness of thedemagnetization curve and the coercivity would decrease significantly.Cylindrical magnets currently used in vibrating motors for cellphones onthe market have an outside diameter of about 2.5 mm, an inside diameterof about 1 mm, and a length of about 4 mm. And their volume isapproximately 16.5 mm³. Thus, those magnets have a surface area tovolume ratio of at least 2 mm⁻¹.

If the volume of magnets keeps on decreasing in this manner, the surfacearea to volume ratio will soon exceed 2 mm⁻¹ and may possibly go beyond3 mm⁻¹ in the near future. The surface reformation of the presentinvention is very effectively applicable to those ultra small magnets.

The Nd—Fe—B based magnets currently used in vibrating motors forcellphones on the market have a (BH)_(max) of approximately 240 kJ/m³,while the magnet according to a preferred embodiment of the presentinvention realizes a (BH)_(max) of 280 kJ/m³ or more, e.g., as high as300 to 360 kJ/m³.

Hereinafter, specific examples of preferred embodiments of the presentinvention and comparative examples will be described.

EXAMPLE 1

A block of a sintered magnet represented by the compositional formulaNd_(29.3)—Dy_(2.0)—Fe_(67.7)—B_(1.0) (where subscripts indicate masspercentages) was made and then machined, thereby making rectangularparallelepiped rare-earth magnet materials with dimensions of 6 mm×4mm×2 mm (which will be referred to herein as “magnet test pieces”). Themagnetization direction of these magnet test pieces was parallel to theside with a length of 6 mm.

The magnet test pieces were loaded into the chamber of a hollow cathodeion plating system to deposit a Ti film to a thickness of 2 μm on thesurface of the magnet test pieces. Specifically, the Ti film wasdeposited under the conditions including a deposition process time of 25minutes, a bias voltage of 25 V or more, and a carrier gas flow rate of200 cc/min.

Next, the test pieces were thermally treated at a temperature of 500° C.to 900° C. for one hour, thereby forming reaction layers between themetal film and the magnet body. For the purpose of comparison, the testpieces were also thermally treated at 200° C. and 300° C. for an hour.

FIG. 2 is a cross-sectional view that was drawn based on a TEMphotograph of a sample representing a specific example of the presentinvention. As can be seen from FIG. 2, in this specific example, anNd-rich layer (first reaction layer) 121 and a Ti—Fe—B layer (secondreaction layer) 122 were formed between a main phase (Nd₂Fe₁₄B crystalgrains) 110 and a Ti film 120. The Nd concentration of the firstreaction layer 121 was higher than that of the main phase 110 and theconcentration of Nd increased there. On the other hand, in the secondreaction layer 122, Ti, Fe and B (boron) were present at higherconcentrations than in the first reaction layer 121. Fe and boronincluded in the second reaction layer 122 came from the main phase(Nd₂Fe₁₄B crystal grains) 110 of the magnet body. And the boronconcentration of the second reaction layer 122 was higher than that ofthe first reaction layer 121. Boron would have bonded to Fe and Ti toform a boride. These reaction layers 121 and 122 with such a two-layerstructure would have been formed by depositing a metal film 120, whichreacted easily with boron, on the surface of the magnet body andthermally treating it at a temperature of 400° C. or more.

As a comparative example, another sample was made by depositing an Alfilm to a thickness of 2 μm on the surface of the magnet body instead ofthe Ti film 120 and then thermally treating it at 500° C. for an hour.The magnetic properties of this sample representing a comparativeexample were evaluated. As a result, the coercivity did not change butthe loop squareness of the demagnetization curve decreased. When theinterface was observed on a TEM photograph, it was discovered that an Nddepletion layer had been formed in contact with the main phase (Nd₂Fe₁₄Bcrystal grains) and a layer with an increased Nd concentration had beenformed in contact with the Al film. The loop squareness would have beendecreased due to the formation of the Nd depletion layer.

FIG. 3 is a graph showing, based on the data that was collected about asample in which Ti was deposited to a thickness of 2 μm on the surfaceof the magnet test piece described above, how much the coercivityincreased or decreased (ΔH_(cJ)) with the heat treatment temperature.The heat treatment was conducted for 10 minutes. In this case, thecoercivity of a comparative example that was thermally treated at 200°C. was used as a reference. As can be seen from FIG. 3, the coercivityincreased significantly in a heat treatment temperature range of 400° C.to 750° C.

FIG. 4 is a graph showing how much the coercivity of a sample, on whicha Ti film had been deposited to a thickness of 2 μm and which had beenthermally treated at 500° C. for 10 minutes, increased or decreased(ΔH_(cj)) with the surface roughness Ra of the magnet body on which theTi film had not been deposited yet. The reference value of coercivitywas 1,175 kA/m. As can be seen from FIG. 4, the smaller the surfaceroughness Ra, the greater the coercivity. The surface roughness Ra ispreferably 0.5 μm or less, more preferably 0.1 μm or less.

EXAMPLE 2

Blocks of sintered magnets represented by the compositional formulaNd_(30.1)—Dy_(1.2)—Fe_(67.7)—B_(1.0) (where subscripts indicate masspercentages) were made and then machined, thereby making thin-platemagnet test pieces with dimensions of 4 mm×6 mm×0.3 mm. Themagnetization direction of these magnet test pieces was parallel to theside with a length of 4 mm.

The magnet test pieces were loaded into the chamber of a hollow cathodeion plating system to deposit a Ti film to a thickness of 2 μm on thesurface of the magnet test pieces as in the first example describedabove.

For a sample representing a specific example of the present invention inwhich reaction layers were formed by depositing a Ti film and thenthermally treating it at 500° C. for an hour and for samplesrepresenting comparative examples in which no reaction layers wereformed at all, the degrees of dependence of the magnetization on theexternal magnetic field were measured to obtain the demagnetizationcurve shown in FIG. 5. The samples in which no reaction layers wereformed include a sample in which a Ti film was deposited but notthermally treated and a sample in which no Ti film was deposited. InFIG. 5, the bold curve shows the results of measurement on a sample thatwas thermally treated (as a specific example of the present invention),the one-dot chain curve shows the results of measurement on a samplethat was not thermally treated at 500° C. for an hour (as a comparativeexample), and the dashed curve shows the results of measurement on asample that had just been machined before reaction layers were formedthere.

As can be seen from FIG. 5, the stepwise decrease after the machiningprocess disappeared in this specific example and the loop squarenesscould be recovered and was better than the comparative examples.

EXAMPLES 3 TO 9

As in the second specific example described above, blocks of sinteredmagnets represented by the compositional formulaNd_(30.1)—Dy_(1.2)—Fe_(67.7)—B_(1.0) (where subscripts indicate masspercentages) were made and then machined, thereby making thin-platemagnet test pieces with dimensions of 4 mm×6 mm×0.3 mm. Themagnetization direction of these magnet test pieces was parallel to theside with a length of 4 mm.

The magnet test pieces were loaded into the chamber of a hollow cathodeion plating system to deposit metal films (shown in the followingTable 1) to a thickness of 2 μm on the surface of the magnet test piecesas in the second specific example described above.

TABLE 1 Sample Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Metal film V ZrNb Mo Hf Ta W Heat 500° 500° 600° 650° 500° 700° 800° treatment C. C. C.C. C. C. C. temperature

For samples representing Examples #3 through #9 of the present inventionin which reaction layers were formed by depositing a metal film and thenthermally treating it at 500° C. for an hour and for samplesrepresenting comparative examples in which no reaction layers wereformed at all, the degrees of dependence of the magnetization on theexternal magnetic field were measured. The demagnetization curvesobtained for the respective samples are shown in FIGS. 6 to 12. Thesamples in which no reaction layers were formed include a sample inwhich one of the metal films shown in Table 1 was deposited but notthermally treated and a sample in which no metal film was deposited. InFIGS. 6 to 12, the bold curve shows the results of measurement on asample that was thermally treated at the temperature shown in Table 1for one hour (as a specific example of the present invention), theone-dot chain curve shows the results of measurement on a sample thatwas not thermally treated (as a comparative example), and the dashedcurve shows the results of measurement on a sample that had just beenmachined before reaction layers were formed there.

As shown in FIGS. 6 through 12, the properties obtained in Examples #3through #9 in which various metal films shown in Table 1 were depositedwere as good as what was obtained in Example #2. That is why a metalfilm according to the present invention does not have to be made of Tibut may also be made of at least one metal selected from the groupconsisting of Ti, V, Zr, Nb, Mo, Hf, Ta and W or an alloy thereof.

EXAMPLE 10

A magnet body represented by the compositional formulaNd_(31.3)—Fe_(67.7)—B_(1.0) (where subscripts indicate mass percentages)was made so as to have dimensions of 7 mm×7 mm×7 mm. After the surfaceof this magnet body was cleaned with a mixed aqueous solution of anitrate salt and nitric acid, a Ta film was deposited to a thickness of3 μm on the surface of the magnet body by an arc ion plating process.Thereafter, the magnet body was thermally treated at 450° C. for onehour, thereby forming reaction layers.

The magnetic properties were measured. As a result, the remanence B_(r)was 1.6 T and the coercivity H_(cJ) was 1,500 kA/m. Thereafter, thismagnet was subjected to a salt-spray test (compliant with the JIS Z 2371standard) using a 5% NaCl solution (with a pH of 7.0) at 35° C. to seewhether the magnet got corroded or not. As a result, even 300 hoursafter the test was started, the magnet never corroded and the magneticproperties thereof never deteriorated to such an extent as to cause someproblems in practice.

Preferred embodiments of the present invention provide an ultra smallrare-earth permanent magnet that can avoid degradation in magneticproperties, which is probably caused by the absence of the grainboundary phase on the surface, and that can keep good corrosionresistance even under a harsh environment.

It should be noted that the present invention is broadly applicable toany type of magnet, not just the sintered magnet, as long as the magnettends to have deteriorated magnetic properties due to the absence of thegrain boundary phase on the surface.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A method for producing a rare-earth sintered magnet, the methodcomprising the steps of: providing a magnet body made of an R—Fe—B basedrare-earth sintered magnet material, where R is at least one rare-earthelement; depositing a metal film on the surface of the magnet body; anda heat treatment process step that is performed to form, between themagnet body and the metal film, a plurality of reaction layersincluding: a first reaction layer, which contacts with at least some ofR₂Fe₁₄B type crystals included in the magnet body, to have received therare-earth element that has been included in the R₂Fe₁₄B type crystals;and a second reaction layer, which is located between the first reactionlayer and the metal film and which has a lower rare-earth elementconcentration than that of the first reaction layer; wherein the secondreaction layer has received boron that has been included in the R₂Fe₁₄Btype crystals and has a higher boron concentration than that of thefirst reaction layer.
 2. The method of claim 1, wherein the rare-earthelement concentration of the first reaction layer accounts for at leastabout 30 mass % of the entire composition of the first reaction layer.3. The method of claim 1, wherein the first reaction layer has athickness of at least about 10 nm.
 4. The method of claim 1, wherein themagnet body has a surface area to volume ratio of at least about 2 mm⁻¹and a volume of at most about 100 mm³.
 5. The method of claim 1, whereinthe surface of the magnet body includes a surface area that has beenformed by a machining process and that is covered with the metal film.6. The method of claim 1, wherein the metal film is made of at least onemetal that is selected from the group consisting of Ti, V, Zr, Nb, Mo,Hf, Ta and W or an alloy thereof.
 7. The method of claim 1, wherein thesurface area of the magnet body has a surface roughness Ra of about 0.5μm or less.